Ideally, a controllable RF phase shifter should have minimum size, minimum insertion loss, minimum weight, minimum cost and complexity, substantial immunity from all adverse ambient environmental factors (including physical and electrical) and an ability to produce any desired phase shift accurately and instantly upon demand. Unfortunately, in spite of many years of effort by those in the art, the truly ideal phase shifter has yet to be realized.
One figure-of-merit commonly used for comparing phase shifter designs is the differential phase shift produced per decibel of insertion loss (.DELTA..phi./dB). Previous ferrite phase shifters of the meanderline and slotline "planar" configuration (e.g. usable as part of a microstrip circuit) have had figure-of-merit factors on the order of 125 for operation in the X-band frequency range. Sometimes diode phase shifters are used in a form of planar substrate phase shifter (e.g. to switch in/out additional microstrip transmission line or to change the reactance across a transmission line). However, such diode phase shifters only have a figure-of-merit on the order of 180 at X-band.
A waveguide mode twin slab ferrite phase shifter (e.g. of the type described in commonly assigned U.S. Pat. No. 4,445,098 - Sharon et al) is one of the most accurate phase shifters known to date. However, in prior realizations, such waveguide mode phase shifters are large and expensive. If unswitched reciprocity is desired, this waveguide unit used in conjunction with circulators is too large for two dimensional phased arrays (where inter-radiator dimensions on the order of 0.6 wavelength are involved). A pair of hybrid mode devices of this invention, however, can be used to realize a non-switched reciprocal phase shifter which will fit the required small dimensions as described in the aforementioned related Roberts application.
At least two types of "planar" ferrite phase shifters have been used in the prior art. The meanderline and the slotline phase shifter are both low cost and lightweight planar ferrite phase shifters. However, high insertion loss and low power handling capability have made both of these devices impractical for general use. As mentioned earlier the figure-of-merit is on the order of only 125 for either the meanderline or the slotline phase shifter. The peak typical power handling capability of these devices (when having a figure-of-merit of 125) is on the order of 1OW to 20W (which is an order of magnitude less than the hybrid mode phase shifter of this invention).
The most common type of meanderline phase shifter has holes in the substrate for a latching wire which carries magnetizing current. For practical nonreciprocal phase shifters there exists a plane where the RF magnetic field is circularly polarized. The imposed phase shift inducing magnetization must be on the axis of the spinning RF magnetic field. The magnitude and direction of this magnetization causes a change in the permeability tensor and therefore a phase change. The meanderline phase shifter basically has a cross-section with a plane in the ferrite substrate where the coupled RF H-fields are orthogonal to each other. The meanderline section is a quarter wavelength long which means, on the axis of the meander, the H fields are orthogonal and one is delayed by 90.degree. referenced to the other. Therefore a circularly polarized H field exists. For this reason the plane of circular polarization exists down the center of the meander section. As one deviates from the meander axis, the wave polarization becomes elliptical and linear at the edges. Therefore the active phase shifting area is only down the axis of the meander. For this reason, and also because of the required high RF currents due to the coupled structure, this device has a low figure-of-merit.
The slotline phase shifter gets its name from the wave structure itself. The slotline phase shifter is a transmission line consisting of a slot in a conductor on a ferrite substrate. The dominant mode in this type of transmission line is similar to a TE.sub.10 mode in rectangular waveguide. The RF magnetic field has a plane of circular polarization in the ferrite substrate. This plane exists where the transverse H field is equal to the longitudinal H field. This phase shifter is not very efficient due to the RF field being distorted at portions extending away from the slot. The most active region is directly below the slot. The fields extending out of the transmission line also contribute to poor figure-of-merit thus making it less useful.
Some prior art patents presently considered relevant to this invention are listed below:
Of those references, Freibergs appears to be possibly the most relevant to a "planar" microstrip phase shifter. However, he leaves the microstrip transmission line intact and simply surrounds it with suitable ferrites, magnetic fields, etc. The Freibergs device has a very low figure of merit (less than 100) and is therefore not very useful for most applications. The Braginski et al, Buck et al, DeLoach and Harris et al approaches to microstrip or stripline phase shifters also appear to leave the transmission line in an uninterrupted status through the phase shifting region (this appears to be true even for Harris et al which also refer to their phase shifter as being a "waveguide" phase shifter).
Landry teaches a waveguide phase shifter having a direct coaxial transmission line to waveguide transition. He notes that a traditional coax-to-waveguide E-plane transition for an unloaded waveguide involves a probe continuation of the coax center conductor extending into the waveguide perpendicular to one of its broad sides at one-fourth wavelength from a short circuit waveguide termination.
Landry then explains why that approach is impractical for phase-shifter waveguides loaded with ferrites and non-homogenous high dielectric structures and that therefore the prior art coax coupling to waveguide phase shifters typically has involved an extra waveguide transformer stage (referring to U.S. Pat. No. 3,758,886 - Landry et al).
Landry notes the lack of space efficiency involved in such prior art extra waveguide sections and then teaches a direct coax-to-waveguide phase shifter transition which includes an E-plane waveguide probe positioned significantly laterally off-center in the dielectric body in a slot extending into its lateral surface. As will be appreciated, effecting such a coupling in ultra-miniaturized waveguide phase shifters would be cumbersome at best.
In addition to Sharon et al, there are also many other examples of various kinds of waveguide ferrite phase shifters including various forms of dual toroid, nonreciprocal, latchable versons. As one simple nonexhaustive exemplary listing, the following are noted:
Some of these have added relevance for various specific details as well. For example, Mason et al teaches dielectric impedance transformers per se. while Dischert teaches metalized ferrite phase shifter structures (as does Birch et al).